1. Field of the Invention
This invention pertains generally to ultrasonic measuring instruments, with particular relevance to ultrasonic flowmeters.
2. State of the Art
An ultrasonic measuring instrument typically comprises sending and receiving transducers, which are placed on opposite sides of an object or material for which a measurement is to be made. Changes that occur in certain characteristics of ultrasonic pulses transmitted through the object or material from one transducer to the other can provide information indicative of certain properties of the object or material. Such information is quantified form can provide useful measurements.
By way of example, ultrasonic flowmeters are commercially available for measuring changes in flow rate of a fluid flowing through a conduit. Sending and receiving transducers positioned on opposite sides of the conduit enable ultrasonic pulses to be transmitted through the flowing fluid from one transducer to the other. The transducers may be multiplexed to reverse the direction of the pulses according to the requirements or capabilities of the particular instrument. Changes in time-of-flight of the ultrasonic pulses from one transducer to the other through the flowing fluid can be correlated with changes in flow rate of the fluid in accordance with well-known principles, a discussion of which can be found in Flow, Its Measurement and Control in Science and Industry, Volume 1, pages 897-957, written by J. L. McShane et al. and published in 1974 by Instrument Society of America, Pittsburgh, Pennsylvania.
Many objects and materials attenuate to a considerable extent the energy in ultrasonic pulses passing therethrough or reflected therefrom. When ultrasonic pulses are transmitted through a fluid, the fluid itself and/or suspended solids carried by the fluid can oftentimes severely attenuate the energy in the pulses. If attenuation of ultrasonic pulses passing through or reflected from a medium is severe enough, coherent pulse signals cannot be detected. The precision with which an ultrasonic measuring instrument can measure small changes in a particular variable is generally dependent upon the sensitivity of the instrument in detecting coherent pulse signals transmitted through or reflected from the medium being investigated.
With ultrasonic measuring instruments of the prior art, the adverse effect of energy attenuation in pulses transmitted through or reflected from a medium was usually overcome by either increasing the energy of the pulses emitted into the medium, or increasing the sensitivity of electronic amplification means for detecting the pulses transmitted through or reflected from the medium. The use of high-gain receiving amplifiers in circuits employing receiving transducers for detecting highly attenuated ultrasonic pulses transmitted through or reflected from objects and materials was a well-known expedient in the prior art.
Historically, the high-gain limit of a receiving amplifier for an ultrasonic measuring instrument was determined by the electronic noise generated by the power supply used to activate the instrument.